The mechanism of interferon induction by UV-irradiated reovirus

Abstract

When reovirus is irradiated with UV-light, its ability to induce interferon in rodent cells increases by a factor of about 200; for the group C mutant ts447 irradiation with UV-light increases its ability to induce interferon at 38° by a factor of more than 10 4. Titers of more than 5×10 6 international units of interferon/10 7 cells are readily achieved. The mechanism that causes UV-irradiation to become such a potent inducer of interferon has been investigated. Incomplete transcripts of reovirus ds RNA segments terminated at the site of a UV-hit were shown to be very unlikely candidates for interferon inducers since they are only formed in very small amounts and the dose-response relationships between UV-dose and synthesis of such incomplete transcripts on the one hand and ability to induce interferon on the other hand are quite different. By contrast, UV-irradiation has a profound labilizing effect on the inner reovirus capsid shell, as evidenced by developing inability of cores to resist digestion by chymotrypsin, accessibility of virion RNA to ribonuclease, and lability to concentrated salt solutions such as CsCl. These in vitro observations were shown to parallel the situation in vivo, where increasing doses of UV-irradiation caused increasing amounts of the dsRNA of infecting virus particles to be liberated into the interior of the cell. No doubt this was due to the increasing instability of the subviral particles to which parental reovirions are converted soon after infection. The dose-response relationships between UV-dose and amount of parental dsRNA liberated into the interior of the cell on the one hand and ability to induce maximal amounts of interferon on the other were the same. Reconstruction experiments with naked dsRNA showed that unirradiated and UV-irradiated dsRNA were equally potent as interferon inducers. It was also whown that neither reovirus cores, nor reovirus top component particles (empty virus particles that lack RNA) can induce interferon in either the unirradiated or UV-irradiated state. It was concluded that UV-irradiated reovirus is a very potent inducer of interferon by virtue of the fact that UV-irradiation labilizes its inner capsid shell and causes some of the dsRNA in parental virus particles to be liberated into the interior of the cell. The mechanism by which such liberated intracellular dsRNA induces interferon was not investigated; however it was noted that the UV-dose that converted reovirions into optimal interferon inducers also caused them to produce maximal cytopathic effects. The relationship between the development of cytopathic effects and the induction of interferon remains to be defined.